Salts having a melting point of less than 200° C., in particular a melting point of less than 100° C., are referred to as ionic liquids. Ionic liquids which are liquid at room temperature are of particular interest. It was believed for a long time that such ionic liquids could not be distilled, since it was assumed that they ultimately have no vapor pressure.
In February 2006, Martyn J. Earle, Jose M. S. S. Esperanca et al published an article about the distillation of volatile ionic liquids in a bulb tube apparatus in Nature, Vol. 439, 2006, pages 831 to 834. However, ionic liquids comprising halides, sulfates or carboxylates decompose and could not be distilled.
WO 2005/068404 describes the distillation of ionic liquids, including those having halides and acetates as anion. A significant aspect here is that the ionic liquids can, owing to an equilibrium reaction, also be present as neutral compounds, i.e. not as salt. As a result of the distillation, these neutral compounds are removed. Continual restoration of the equilibrium state results in the entire ionic liquid distilled in the form of the neutral compounds. In the case of ionic liquids having nitrogen-comprising, heterocyclic ring systems as cation and, for example, halides or carboxylates as anion, a corresponding equilibrium state can be established if at least one nitrogen atom of the ring system is not substituted by an organic group and is thus available for an equilibrium reaction of the anion. Accordingly, only the chlorides of 1-ethylimidazole or 1-methylimidazole are distilled in the examples of WO 2005/068404.
Douglas R. MacFarlane, Jennifer M. Pringle et al., Chem.Commun., 2006, pages 1905 to 1917, also disclose a distillation of ionic liquids. Here, the ability to be distilled is based on an equilibrium reaction in which the cation and anion of the ionic liquid are present as neutral acid and base. As indicated above, the neutral compounds are withdrawn from the equilibrium state and distilled. In this way, it is possible to distill imidazolium acetates in which a nitrogen atom of the heterocyclic ring system is present in protonated form (HMIM acetate in Table 4 of the article).
Ionic liquids are generally not consumed but only contaminated during use. Since they are high-priced materials, there is a need for particularly effective and advantageous methods of working up and separating the anionic liquids from the mixtures obtained in use. When ionic liquids are used for dissolving cellulose, mixtures comprising lignins or cellulose derivatives, for example, are formed. Furthermore, inexpensive processes for preparing ionic liquids are known, but these form relatively nonvolatile by-products, the reaction products obtained are discolored because of these by-products and generally appear black. Such processes are described, for example, in WO 2005/021484 (carbonate method) or in WO 91/14678 (Arduengo process). Here too, there is a need for particularly effective and advantageous methods of working up and separating the ionic liquids from the mixtures obtained in the preparation.
It is therefore an object of the present invention to provide a simple and effective method of purifying or working up ionic liquids or the mixtures obtained in their preparation and/or use.
We have accordingly found the method defined at the outset.